The present invention relates to the field of fiber reinforced metal type composite materials, and more particularly relates to the field of such fiber reinforced metal type composite materials which include alumina or carbon fibers as reinforcing material, or mixtures thereof, and which utilize aluminum alloy as the matrix metal.
In motor vehicles and aircraft and so forth, nowadays, the constant demand for lightening and strengthening of structural members and parts has meant that construction from aluminum has become common. Problems arise, however, in making parts from aluminum or aluminum alloys, despite the light weight of these aluminum alloys, and despite their easy workability, because the mechanical characteristics of aluminum alloys such as strength, including bending resistance, torsion resistance, tensile strength, and so on are inferior to those of competing materials such as steel. Further, the occurrence of cracking and the spreading of cracks in parts made of aluminum alloy can be troublesome. Therefore, for parts the strength of which is critical there are limits to the application of aluminum alloys.
Accordingly, for such critical members, it has become known and practiced for them to be formed out of so called two phase or composite materials, in which reinforcing material is dispersed within a matrix of metal. If the matrix metal is aluminum alloy, then the advantages with regard to weight and workability of using this aluminum alloy as a constructional material can be obtained to a large degree, while avoiding many of the disadvantages with regard to low strength and crackability; in fact, the structural strength of the composite materials made in this way can be very good, and the presence of the reinforcing material can stop the propagation of cracks through the aluminum alloy matrix metal. The reinforcing material conventionally has been known as for example being alumina fibers, or carbon fibers, or a mixture thereof, and the matrix metal has been known as for example being various types of aluminum alloy; and various proposals have been made with regard to compositions for such fiber reinforced metal type composite materials, and with regard to methods of manufacture thereof. A brief discussion of these types of composite materials, and their methods of manufacture that have been developed by various companies, and of the related aluminum alloys that are used for the matrix metal thereof, will now be given.
1. THE HIGH PRESSURE CASTING METHOD
First a mass of reinforcing fibers is placed in the mold cavity of a casting mold, and then a quantity of molten aluminum alloy is poured into the mold cavity. The molten aluminum alloy matrix metal is then pressurized to a high pressure such as approximately 1000 kg/cm.sup.2 by a plunger or the like, which may be slidingly fitted into the mold. Thereby the molten matrix metal is intimately infiltrated into the interstices of the mass of reinforcing fibers, under the influence of this pressure. This pressurized state is maintained until the aluminum alloy matrix metal has completely solidified. Then finally, after the aluminum alloy has solidified and cooled into a block, this block is removed from the casting mold, and the surplus aluminum alloy around the reinforcing fibers is removed by machining, so that the composite material mass itself, consisting of the mass of reinforcing fibers impregnated with aluminum alloy matrix metal, is isolated. This high pressure casting method has the advantage of low cost, and it is possible thereby to manufacture an element of a relatively complicated shape with high efficiency.
With regard to this high pressure casting method, as is described in Japanese patent application No. Sho 55-107040 (1980), the reinforcing material fiber mass may be preheated to a substantially high temperature of at least the melting point of the aluminum alloy matrix metal, before the matrix metal is poured into the casting mold, in order to aid with the proper penetration into and proper impregnation of the reinforcing material fibers by the matrix metal. Further, as is described in Japanese patent application No. Sho 56-32289 (1981), the reinforcing material fiber mass may be, before the casting process, charged into a case of which only one end is left open, an air chamber being left between the reinforcing material fiber mass and the closed end of the stainless steel case, and then the case with the reinforcing fiber mass therein may be placed into the mold cavity of the casting mold, and pressure casting as described above may be carried out. This concept of utilizing a case with an air chamber being left therein again serves to aid with the proper penetration into and proper impregnation of the reinforcing material fibers by the matrix metal, and more details will be found in the above identified Japanese patent application, if required.
In this high pressure casting method, a typical aluminum alloy used is JIS (Japanese Industrial Standard) type AC8A, which is approximately 12.0% silicon, 0.8% copper, 1.2% magnesium, 2.5% nickel, and the remainder aluminum. Another possibility is JIS AC8B, which is approximately 9.5% silicon, 3.0% copper, 1.0% magnesium, 1.0% nickel, and the remainder aluminum; and another is JIS AC4C, which is approximately 7.0% silicon, 0.3% magnesium, and the remainder aluminum. Various other possibilities are also employed.
2. THE METHOD OF THE COMPANY FIBER MATERIAL INC.
This method is performed as follows. First, onto the surfaces of carbon fibers titanium and/or boron is applied by chemical evaporation deposition, and then these fibers are dipped into molten aluminum alloy, thus forming a preimpregnated mass, since the fibers are thus precoated with aluminum alloy. Next, a number of layers of this preimpregnated mass are sandwiched together and sintered. The production cost of this preimpregnation method for producing a composite material is high, as compared with the cost of the above described high pressure casting method, and there are other defects inherent therein, such as the fact that the volume ratio of the reinforcing fibers cannot be made very high, and also that is it not possible to manufacture elements of complicated shapes such as for example cylinders.
In this dipping type preimpregnation method, a typical aluminum alloy used is AA standard A201, which is approximately 0.1% silicon, 4.7% copper, 0.3% magnesium, 0.6% silver, and the remainder aluminum. Another possibility is AA standard A356, which is approximately 7.0% silicon, 0.2% copper, 0.3% magnesium, and the remainder aluminum; and another is AA standard A6061, which is approximately 0.6% silicon, 0.25% copper, 1.0% magnesium, 0.2% chromium, and the remainder aluminum. Various other possibilities are also employed; these are all general purpose type aluminum alloys and rolling aluminum alloys.
3. THE METHOD OF THE COMPANY TOHO BESURON K.K.
This method is performed as follows. First, onto the surfaces of carbon fibers aluminum alloy is deposited by physical evaporation deposition, thus forming a preimpregnated mass, since the fibers are thus precoated with aluminum alloy. Next, a number of layers of this preimpregnated mass are sandwiched together and hot pressed together. The production cost of this preimpregnation method for producing a composite material is also high, as compared with the cost of the above described high pressure casting method, and there are again other defects inherent therein, such as the fact that the volume ratio of the reinforcing fibers cannot be made very high, and also that it is not possible to manufacture elements of complicated shapes such as for example cylinders.
In this evaporation type preimpregnation method, the aluminum alloy generally used is AA standard A5056, which is approximately 0.3% silicon, 0.1% copper, 4.5% to 5.6% magnesium, 0.4% iron, 0.05% to 0.2% manganese, 0.05% to 0.2% chromium, 0.1% zinc, and the remainder aluminum. This aluminum alloy is generally used because it has good wetting ability in conjunction with carbon fibers and is suitable for diffusion bonding.
4. THE METHOD OF THE DUPONT COMPANY
In this method, a mass of reinforcing material in the form of alumina fibers is fitted into a stainless steel case of tubular form which is open at both ends, and then one end of the case is dipped into molten aluminum alloy, while the pressure at the other end of the case is reduced by sucking, so that the aluminum alloy is sucked up and is caused to impregnate between the alumina fibers. In this method, reuse of the stainless steel case is difficut, which increases the cost of production, and also in order to have good wetting ability of the alumina fibers by the molten aluminum alloy matrix metal it is necessary to add a certain amount of lithium to the molten aluminum alloy. Since such lithium is expensive, this further undesirably increases the production cost, thus resulting in a high cost fiber reinforced metal composite material product.
In this sucking impregnation type of method for making composite material, the aluminum alloy generally used is an aluminum alloy containing about two to three percent lithium and the remainder aluminum; if the lithium content is greater than about three percent, then the alumina fibers deteriorate, whereas if the lithium content is less than about two percent the aluminum alloy does not well wet the alumina fibers and penetrate between them into their interstices to impregnate them. For these reasons, maintaining the lithium content of the aluminum alloy in this tight range is important, and this is difficult. This further increases the cost of the resulting composite material.
5. OTHER METHODS
Other methods such as powder metallurgy methods are known for making such a fiber reinforced metal composite material, and in these methods the aluminum alloy used is generally a general purpose rolling aluminum alloy, such as AA standard A6061 or AA standard A2024.